Out of the blue, Treyarch hired B-movie legends like Sarah Michelle Gellar, Robert Englund, Michael Rooker and Danny Trejo -- all posing as themselves -- and pitted them against the unrelenting waves of the undead, and even brought zombie master George Romero into the mix as a boss. The Zombie mode itself has always been presented as tongue-in-cheek fan service (the original mode from World at War had players fighting Nazi Zombies, and maps since then have included zombie monkeys and mad scientists), but "Call of the Dead" takes the goofiness even further.

So, how exactly did this happen? Joystiq cornered Zombies creative lead Jimmy Zielinski at a press event for the map pack earlier this week, and asked him how the developer decided to use some of the most familiar names in horror and monster film history for just one map in an add-on pack.

The origins of Call of the Dead actually lie in another pack for Black Ops' Zombie mode: the secret "Presidents" map that was unlocked at the end of the single-player game. That was the first time Treyarch put a little personality into the player characters. "The first map that ever shipped for Zombies had no player characters, they were all soldiers," Zielinski says. "So this was the first departure from that. And in response to the community and user testing, we just realized how popular it was, we kind of thought already, 'When are we going to do this again and what is it going to be?'"

"You start throwing around like who would it be fun to play as," he continues. "I think one of the first things even was to have a female character." Zielinski says the development team made a list of who they'd like to have in the map, and a few certain choices ended up bubbling to the top. "Honestly, we were really looking at the top five and going yeah, those would be nice, and then we got them. It just kind of worked out that way."

It didn't hurt that Activision's Call of Duty series is one of the biggest names in gaming. While Zielinski didn't work specifically with the actors (his team is gameplay-oriented, and there were other teams that recorded dialogue and acting), he says they all "had already either heard of the game or had a first-hand experience with it."

George Romero was quite a catch as well -- not only does the legendary zombie director lend his testy personality as an actor, but he appears as a zombie boss as well. Zielinski says the profound honor of portraying Romero in his own genre wasn't lost on the team. "Call of the Dead is, simply put, Treyarch's homage to the granddaddy of zombies. His movies have served as such an inspiration to us," he says. The results of this casting were apparently worth the arguments on the dev team, too. "We're all really happy with our choices and their performances, and it sounds like the fans are responding as well."

Call of the Dead, like all of the past maps, changes up the gameplay for the Zombies mode -- while the secret Presidents map was intended to be a little tougher and intended for players who wanted a challenge, this new map was designed to expand Treyarch's arsenal of mechanics. "Our primary objective has always been to push the boundaries of the Zombies mode," says Zielinski. "So in one hand we have this habit of introducing new features like zombies, weapons, perks, traps and mechanics with every new map and we will continue to do that.

"On the other hand, we also like to design our maps' layout in ways to encourage completely new styles of gameplay."Call of the Dead makes use of some of those new styles, allowing players to do a little sniping, or play around with Romero's calm and berserk modes as a boss character.

The Zombie mode itself is obviously a big part of Treyarch's side of the Call of Duty franchise -- Zielinski calls it one of "the three legs" of the game, with multiplayer and the single-player campaign making up the other two. Strangely, Zielinski says that Treyarch doesn't actually have data on how popular the maps are compared to the more traditional modes, but that they're important for both the team and the franchise nevertheless.

"We look at our community," he says. "We look at how feverish they are about the game and how involved and jazzed up about them coming [out they are]. I don't know that we have actual data to divide the pile or anything like that but it is seen as very important. And it's given backing as such."

Still, Zielinski plays coy on the future of the Zombies mode. Obviously Treyarch is working on something else, but whether that's just another map pack, another iOS game featuring the Zombies mode, or maybe even a standalone Zombies title for consoles, Zielinski isn't saying one way or the other. "We haven't announced any future plans for anything yet," he says.

The only hint we can get out of him, in fact, is when I ask if Treyarch is thinking about bringing any other actors or characters into the mode in the future. "What I would say is that I don't think there's anything off the table. For Zombies in particular ... the ideas that we're able to come up with and the resources that we're able to pull from are just unlimited."

Is there a chance we'll see these characters again, Buffy and Danny Trejo and Freddy Krueger, fighting another zombie outbreak? "Stay tuned," is all Zielinski can say.